SH3100 Datasheet, PDF (66/80 Pages) Semtech Corporation – Supervisory IC with I2C Interface and PWM

SH3100
POWER MANAGEMENT
Functional Descriptions (continued)
PWM Operation & Fan Speed Control
Auto-PWM Engine Features:
The PWM function is implemented as a 10-bit PWM coun- _{• 7-bit minimum temperature setting register, which de-}
ter clocked at the CLK1 rate. Therefore, if CLK1 is set to
fines, with 2°C resolution, the temperature at which
32.768 kHz, the PWM repeat rate is 32 Hz.
the PWM comes on. The state machine incorporates
The PWM duty cycle can either be set directly by writing to
the ForcePWMDutyCycle register or may be programmed
to track the temperature in AutoPWMMode.
4°C of hysteresis to prevent the fan from continually
switching on & off when the temperature is approxi-
mately the minimum.
• 10-bit minimum duty cycle register (shared with the
The PWM function normally operates in standard PWM
ForcePWMDuty when not in AutoPWMMode), which
output format, but it can be programmed to operate in
defines the starting duty cycle.
PDM (Pulse Density Modulation) mode, which outputs the
same energy density, but distributed more evenly over the • 5-bit DutyCycleStepSize register, which determines
PWM cycle. This is useful when the PDM output is going
the duty cycle increments per °C. This register along
to be filtered to generate a DC level.
with the MinTemp register determines the maximum
temperature.
Auto-PWM Mode (Fan Control Mode)
• The PWM starts at 100% duty cycle for between 1 and
This is an application of the PWM function which oper-
ates in conjunction with the general purpose 8-bit DAC
2 seconds to kick-start the motor, before settling to
MinDutyCycle. Thereafter it tracks with temperature.
and comparator to provide automatic fan speed control.
It can be enabled by setting the SMPS mode bits in the ^{• If the SNSE input pulses cease, the PWM generates}
registers such that operation commences immediately on
100% duty cycle for 1 or 2 seconds, and if there are
power-up. The external application circuitry is shown in
still no pulses, enters the FAULT state and generates
operating mode 3.
an interrupt.
AutoPWMMode has been designed as a flexible fan con- ^{• If the duty cycle reaches 100% due to the maximum}
trol engine for brushless DC motors, providing a PWM duty
temperature being reached, an interrupt is generat-
cycle proportional to the temperature. When in AutoPWM-
ed.
Mode, temperature is measured once per 32 PWM cycles _{• Software can read the AutoPWMDutyCycle register to}
which means once per second at the power-up default
determine the current duty cycle is for diagnostic pur-
rate of 32 Hz.
poses.
Fan speed control uses the SNSE input to detect when _{• The PWM counter is clocked from the same source as}
the fan has stalled. While the fan is running, voltage tran-
CLK1. The CLK1 output does not need to be enabled.
sients on the SNSE pin crossing the set threshold indicate
In theory, this allows the PWM rate to be varied up
to the controller that the fan is running correctly. If the
to a maximum PWM repeat rate of 32.7 kHz (33.5
SNSE transitions cease for more than 1 second, then this
MHz/1024) , but since the temperature measure-
indicates that the fan has stalled, and the PWM duty cycle
ment interval scales with PWM rate, the ADC conver-
should be set to 100% to try to restart the fan. Both posi-
sion rate or the SNSE pulse repeat rate limits the ef-
tive and negative going transitions are detected, so if the
fective maximum CLK1 frequency.
SNSE signal stops in any state, then fan stall is detected.
© 2006 Semtech Corp.
66
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